While the airway epithelium provides a diffusion barrier to the access of inhaled stimulants, it has recently been considered to have a more active physiological and pharmacological role in the regulation of the airway smooth muscle tone. The initial evidence for this comes from the fact that mechanical removal of the airway epithelium increases the in vitro responsiveness of airway smooth muscle to various spasmogens 3-5 times, although the nature of these regulatory mechanisms on the smooth muscle by the epithelium remains largely unresolved. One of the possible explanations is that the epithelium a generates factor(s) that inhibits the responsiveness of the underlying smooth muscle cells (epithelium-derived inhibitory or relaxing factor EpDIF/EpDRF). Furthermore, recent investigations revealed that airway epithelial cells also release a factor(s) that modulates the vascular smooth muscle tone, excitatory-neuroeffector transmission and resting membrane potential of the airway smooth muscle cells (epithelium-derived hyperpolarizing factor: EpDHF). This review will present and discuss the evidence indicating that the epithelium generates and releases a factor(s) that modulates smooth muscle tone, resting membrane potential of airway smooth muscle cells (EpDHF) and excitatory neuro-effector transmission in the airway.
The quantitative receptor autoradiographic method has been developed to determine the precise location and kinetic properties of receptors for biologically active transmitters, besed on the original technique for benzodiazepine receptors in the rat brain with a 3H-labeled ligand. The autoradiography with hyperfilm-3H of cryostat sections (10- to 20-μm thick) incubated with appropriate 3H- or 125I-labeled ligands and analysis of the autoradiogram by computerized microdensitometry makes feasible quantitative determinations and complete characterization of receptors in tissues from a single animal and in human tissues obtained at autopsy. Furthermore, the autoradiographic technique has advantages over receptor binding assays with partially purified membrane preparations because it enables precise anatomical localization and has high sensitivity. In some cases, the autoradiogram can be quantitated with a computerized radioluminographic imaging-plate system. Special care has to be taken when a 3H-labeled ligand is used, since tissue quenching of the radioisotope energy of 3H is linked to the failure to attain precise quantitation of receptors in myelin-rich areas such as brain white matter. In combination with quantitative radioimmunohistochemical and in situ hybridization techniques, the quantitative receptor autoradiographic method should enable investigators to elucidate the roles of receptors for biologically active transmitters.